The present invention relates to a track search apparatus for searching or accessing a desired track from a recording medium having many tracks.
As a prior art apparatus, there has been Proposed an optical type recording and reproduction apparatus, for example.
In the optical type recording and reproduction apparatus, a light beam, which is produced from an optical source such as a semiconductor laser diode, is irradiated and focused through a focusing lens onto an information medium (hereinafter referred to as a disk) which has an optically recordable and reproduceable material film formed on the surface of a substrate having concentric uneven-structured tracks by means of vacuum evaporation. And during the signal reproduction, the light amount thereof is fixed comparatively weakly to do signal read in terms of the light reflected from the disk, whereas during the signal recording, the light amount thereof is modulated (weak or strong) according to the signal to be recorded to do signal write.
In such an optical type recording and reproduction apparatus, focusing control for controlling the light beam to be always in a substantially predetermined focusing state on the recording material film and tracking control for controlling the light beam to be always located on the track are performed. Also in order to randomly access the tracks on the disk by the light beam, track search control is carried out in which deactivating the tracking control, the light beam is shifted in the direction of the disk radius toward a target track and the tracking control is activated again when the light beam arrives at the target track. The prior art relative to the track search is disclosed in U.S. Pat. No(s). 4,106,058, 4,332,022, etc.
One of the important things relative to the track search is the speed when the light beam enters an object track, that is, a tracking pull-in speed. The control frequency for tracking control has a limitation, specifically is usually limited to about several KHz's. For this reason, if the tracking pull-in speed is too high, the pull-in of tracking control into a target track will result in failure; on the other hand if the tracking speed is too low, it will take a long time to perform the track search.
Therefore, in shifting the light beam in the direction of the disk radius in the track search, velocity control of controlling the speed of the light beam is performed in order to control the tracking pull-in speed accurately thereby doing the stabilized pull-in of tracking control into a target track.
The track search is carried out by shifting the light beam in the direction of the disk radius in such a way that the speed of the light beam becomes the predetermined reference velocity corresponding to the present position of the light beam during the track search operation.
The present speed of the light beam required to do the track search is detected by the period of a track traversing signal produced when the light beam traverses a certain track. The present position of the light beam is obtained by counting track traversing signals accumulated from a start track in the track search. FIGS. 2a to 2d show a tracking error signal and a tracking traversing signal produced when the light beam traverses tracks in the direction of the disk radius. Specifically, FIG. 2a shows the manner that the light beam traverses the tracks on the disk and FIG. 2b shows the tracking error signal thus produced.
The tracking error signal can be derived in the manner of FIGS. 2a and 2b by means of the push-pull method for an uneven-structured track with the optical depth of approximately .lambda./8 assuming that the wavelength of the light beam is .lambda.. This is disclosed in detail in Japanese Patent Publication No. 59-9085 (published in Feb. 29, 1984) and French Patent 7529707 (published in Sep. 29, 1975) and hence the explanation will be omitted here.
FIG. 2c shows the waveform of a binary signal into which the tracking error signal was converted and FIG. 2d shows an edge detection signal represented by the detected rising edges of the waveform of FIG. 2c. Each pulse of the edge detection signal is generated when the light beam traverses the center of each track as seen from FIG. 2a, and therefore the edge detection signal represents a track traversing signal. Thus, the value obtained by counting this track traversing signal from the start of track search indicates the present position of the light beam. Moreover, since tracks are formed at substantially regular intervals of P on the disk in the direction of the disk radius, the speed of the light beam, assuming that the period of the track traversing signal is T, can be expressed by EQU V=P/T (1)
FIG. 2e shows the edge detection signal with both detected rising edges and falling edges of the signal of FIG. 2c. Assuming that the period of this edge detection signal is t, the speed of the light beam can be likewise expressed by EQU V=P/2t (2)
However, detecting the speed of the light beam from the period of the track traversing signal as mentioned above makes the speed detection intermittent, thereby producing phase delay due to sampling. Hence, the frequency band for speed control could not be made sufficiently high. Consequently, in the conventional optical type recording and detection apparatus, if in carrying out the track search, the eccentricity involved in the disk is large, or vibration or shock is applied to the apparatus, control error in the speed control could become large and the tracking pull-in speed could also become too high thereby leading to failure of the pull-in of tracking control for an object track; on the other hand, if the tracking pull-in speed is too low, it could take a long time to complete the track search.